The present invention relates to a semiconductor device, and more particularly, to a semiconductor device in which metallic protrusions (hereinafter called "bumps") are provided at an electrode wiring section on a semiconductor chip to make electrical connection with external leads.
Heretofor, electrodes have been formed on a semiconductor chip with a silicon substrate or the like in the following manner. After required circuit elements are formed in a semiconductor chip and the surface of the chip is covered with an insulator film (for example, a silicon oxide film), the insulator film is partially removed by etching at portions where electrical connection to regions of semiconductor chip is required to form apertures. Then, aluminum is deposited over the surface of the chip including the apertures and is partially removed by etching to form a predetermined pattern of wirings. At portions of the wirings where external leads are to be connected, a thin film of titanium, platinum or the like is formed and bumps of highly conductive metal such as gold, silver or the like are formed thereon by plating. The external leads are connected to the bumps by thermo-compression bonding. The intermediate thin film of titanium or platinum is necessary, because if the gold bumps are directly contacted with the aluminum layer without the intermediate thin film, then an alloy layer called "purple plague" would be formed therebetween by chemical reaction between aluminum and gold and cracks would arise therebetween due to volume increment by formation of this alloy which would result in anomalous increase in a resistance or breaking of a circuit.
Various proposals have been made for connection between external leads and bumps, and one of them is disclosed in British Pat. No. 1,196,834. According to this disclosure, aluminum electrodes making electrical contact with operating regions of a semiconductor chip are formed extending onto an oxide film outside of the operating region, and then at an end of the aluminum electrode, a layer of refractory metal such as titanium, chromium, or tungsten, serving as a barrier film for preventing formation of the purple plague is formed so as to overlap the end of the aluminum electrode. Thereafter an oxide film is formed over the surface of the chip, and the predetermined part of the oxide film on the refractory metal layer is selectively removed by etching to make an aperture in which the refractory metal is exposed. A conductive metal layer of conductive metal liable to be wetted with solder (tin-lead alloy), such as a layer of nickel, copper or gold is formed within the aperature. Thereafter, the whole chip is immersed in a solder bath to form a bump of solder on the conductive metal layer within the aperture.
The bump thus formed is in contact with the inner wall of the oxide layer in the aperture. Therefore upon thermo-compression bonding an external lead to the bump, the bump would be deformed by the mechanical force exerted thereon due to the thermo-compression bonding, and it is liable to occur that cracks are produced in the oxide film or the oxide film is torn off around the bump due to the stress associated with the deformation. Consequently, humidity and contamination would enter from external atmosphere into the semiconductor device through the cracks or the torn portions and would corrode the conductive metal as well as the aluminum wiring, which causes accidents such as poor connection or breaking of a circuit or degradation of electrical performances of the device.
According to another proposal in the prior art such as disclosed, for example, in U.S. Pat. No. 3,942,187, an aluminum wiring layer is provided extending from an operating region of a chip onto a first oxide film, and further, a first nickel layer is formed on the oxide film so as to overlap the end of the aluminum wiring layer. Thereafter, a second oxide film layer is formed over the entire surface of the chip and is partially removed from a portion on the first nickel layer to provide an aperture where a second nickel layer is formed within the aperture as well as on the peripheral portion of the aperture. A bump of copper or gold is provided on the second nickel layer so as to cover the aperture. However, even with this structure, the bump would be deformed laterally, when an external lead is bonded thereto, by the pressure exerted upon the bonding, and consequently cracks or tear-off would arise in the second oxide film layer located around the deformed bump due to the stress associated with the deformation. Therefore, it is inevitable that the reliability of the semiconductor device is greatly lowered.